Environmental concerns have led to continued efforts to reduce the NOx emissions of compression ignited (diesel) internal combustion engines. The latest technology being used to reduce the NOx emissions of diesel engines is known as exhaust gas recirculation or EGR. EGR reduces NOx emissions by introducing non-combustible components (exhaust gas) into the incoming air-fuel charge introduced into the engine combustion chamber. This reduces peak flame temperature and NOx generation. In addition to the simple dilution effect of the EGR, an even greater reduction in NOx emission is achieved by cooling the exhaust gas before it is returned to the engine. The cooler intake charge allows better filling of the cylinder, and thus, improved power generation. In addition, because the EGR components have higher specific heat values than the incoming air and fuel mixture, the EGR gas further cools the combustion mixture leading to greater power generation and better fuel economy at a fixed NOx generation level.
Diesel fuel contains sulfur. Even “low-sulfur” diesel fuel contains 300 to 400 ppm of sulfur. When the fuel is burned in the engine, this sulfur is converted to SOx. In addition, one of the major by-products of the combustion of a hydrocarbon fuel is water vapor. Therefore, the exhaust stream contains some level of NOx, SOx and water vapor. In the past, the presence of these substances has not been problematic because the exhaust gases remained extremely hot, and these components were exhausted in a disassociated, gaseous state. However, when the engine is equipped with an EGR system and the exhaust gas is mixed with cooler intake air and recirculated through the engine, the water vapor can condense and react with the NOx and SOx components to form a mist of nitric and sulfuric acids in the EGR stream. This phenomenon is further exacerbated when the EGR stream is cooled before it is returned to the engine.
From the foregoing, it is clear that lubricants for modern heavy duty diesel engines must be able to provide proper performance in a particularly harsh environment.
Concurrent with the development of the condensed EGR engine, there has been a continued effort to reduce the content of sulfated ash, phosphorus and sulfur in the crankcase lubricant due to both environmental concerns and to insure compatibility with pollution control devices used in combination with modern engines (e.g., three-way catalytic converters and particulate traps). A particularly effective class of antioxidant-antiwear additives available to lubricant formulators is metal salts of dialkyldithiophosphates, particularly zinc salts thereof, commonly referred to as ZDDP. While such additives provide excellent performance, ZDDP contributes each of sulfated ash, phosphorus and sulfur to lubricants. The most recent lubricant specifications in each of Europe (ACEA E6) and the United States (API CJ-4 (or PC-10)) require reductions in allowable levels of sulfated ash, phosphorus and sulfur relative to the prior standard, and have required reductions in the amount of ZDDP that can be used. Where reduced amounts of ZDDP are employed, alternative means of providing engine wear protection must be identified, preferably means that do not cause introduction of additional sulfated ash into the lubricant.
Surprisingly, it has been found that lubricating oil compositions employing certain select detergents exhibit excellent antiwear performance in diesel engines, including heavy duty diesel engines provided with EGR systems, using reduced levels of ZDDP.